Bacillus anthracis is classified as one of seven Centers for Disease Control and Prevention category A agents that are considered major threats as bioweapons. B. anthracis is a rod-shaped Gram-positive spore-forming bacterium, which in nature is sometimes found in ruminants. Although natural anthrax infection in humans is rare (risk of infection through contact with diseased animals is about 1/100,000), it poses a very real threat from bioterrorism. The bacteria form hardy spores, which in nature are usually found in the soil. The spores are relatively heat resistant and can survive for decades under suitable conditions.
B. anthracis secretes a toxin (anthrax toxin) which contributes to bacterial virulence and causes many of the disease symptoms. Anthrax toxin is an AB-type toxin composed of a receptor-binding B-moiety and two catalytic A-moieties. The receptor-binding B-moiety is referred to as the protective antigen (PA). The two catalytic A-moieties are lethal factor (LF) and edema factor (EF). LF and PA combine to form lethal toxin, and EF and PA combine to form edema toxin. The receptor-binding PA component of the toxin facilities delivery of EF and LF into the cell by binding to cell surface receptors (called anthrax toxin receptors, ATR or ANTXR). Following entry into the cell in an endosome, heptaermized PA inserts into the endosomal membrane at acidic pH, forming a pore that mediates translocation of the enzymic components of the toxin from the endosomal compartment and into the cytosol. Entry of EF and LF into the cell cytoplasm then leads to cell death.
Anthrax has three primary modes of entering the human body: through the intestines (ingestion), lungs (inhalation), or skin (cutaneous). Although cutaneous anthrax is more readily treatable, inhaled anthrax typically results in an abrupt catastrophic illness having a mortality rate of greater than 80% in 2-4 days. If anthrax spores were spread through an act of terrorism, the event would likely be undiscoverable until large numbers of people sought treatment or died.